In the past, bodies of ferromagnetic material have been inspected by a method such as the leakage flux method as taught, for example, in U.S. Pat. Nos. 3,091,733, May 28, 1963 Fearer et al and 4,602,212, July 22, 1986, Hiroshima et al. In this method, the metal is magnetized in a direction parallel to its surface. At defects or where regions of the metal body are thinner, some magnetic flux passes into the air and may be detected by sensor, thus giving an indication of the presence of faults.
U.S. Pat. No. 4,107,605, Aug. 15, 1978 Hudgell discloses an eddy current technique for testing of pipelines of ferrogmagnetic material. The probe includes spiral sensing coils placed with their axes normal to the surface of the pipeline wall and connected on four legs of an AC bridge, thus compensating for lift-off, Biasing electromagnetic fields permit distinguishing internal from external defects in weakly ferromagnetic tubes by comparing outputs from systems with and without biasing fields.
In U.S. Pat. Nos. 2,992,390, July 11, 1961 Dewitte and 3,940,689 Feb. 24, 1976 Johnson, Jr., special ways of generating magnetic fields are taught in connection with the eddy current testing in that DeWitte uses uniquely designed core and Johnson, Jr. employs a solenoid wound about a core of a substantial length. U.S. Pat. No. 4,292,589 Sept. 29, 1981 Bonner on the other hand teaches the use of unique coil arrangements for a differential receiver of a remote-field eddy current probe. However, his arrangement requires long probes and low test frequency, thus limiting inspection speed. U.S. Pat. Nos. 3,952,315 Apr. 20, 1976 Cecco and 2,964,699 Dec. 6, 1960 Perriam describe probes for use of testing weakly ferromagnetic tubes. They use impedance type sensing circuit but are not sensitive to circumferential cracks nor are they circumferentially compensating either.
All the prior art instruments suffer from various shortcomings such as high cost, requirement of specially designed instrumentation, bulky electromagnetizers for high magnetic saturation, insensitivities for certain kinds of defects, no defect sensitivity close to weld joints etc.